Past growth of conventional powder metal stainless steel depended greatly on the availability of stainless steel powders permitting the economic production of complex shapes with adequate mechanical properties and moderate corrosion resistance.
The current exclusive use of fully prealloyed water atomized stainless steel powders was preceded by the use of elemental powder blends and powders obtained through intergranular corrosion of sensitized or embrittled stainless steel sheets in the 40's and early 50's. With the improvement of the atomization process, atomized stainless steel powders became more popular, and chemical compositions were optimized mainly with respect to compacting properties. The two major improvements consisted of: (1) the lowering of the oxygen contents from typically over 5000 PPM to about 2000 PPM through the use of a protective atmosphere in the atomization chamber, a reduced superheat, and through proper balancing of the manganese and silicon contents; and (2) chemistry optimization of other constituents. Both measures improved green strength, compressability, and sintered properties of the powders.
The effects of sintering parameters, particularly sintering atmosphere, sintering temperature, and part density with respect to mechanical properties of sintered parts are well documented in the literature. (See: Stosuy, "Sintered Type 316 Stainless . . . Its Properties and Processing", Metal Progress 91, 1967, pp. 81-85.; Dautzenberg, "Eigenschaften von Sinterstaehlen aus wasserverduesten unlegierten und fertiglegierten Pulvern", Proc. 2nd European Symposium on Powder Metallurgy, May 8 to 10, 1968, paper No. 6.18.; Sanders, "Stainless Steel P/M Alloys--Unique Applications", 5th Int. P/M Conf. 1976.; Kato, "On Some Properties of Sintered Stainless Steels at Elevated Temperatures", Powder Metallurgy (Jap.) vol. 27, No. 5, 1980, pp. 2-8.
Until recently, studies on the corrosion resistance of conventional Powder Metallurgy (P/M) stainless steel parts were limited to questions dealing with the effects of processing, bulk chemistry modification, and post treatments, and use of higher alloyed compositions. In 1980, Ro and Klar (See: Ro and Klar, "Corrosion Behavior of P/M Austenitic Stainless Steels", Modern Developments in Powder Metallurgy, Vol. 13, 1981, pp. 247-287.) reported the surfaces of water atomized stainless powder to consist of silicon rich oxide film. While the presence of about 0.7% to 1.0% Si in typical stainless steel powders minimizes oxidation during atomization, thereby assuring good compacting characteristics, Ro and Klar found the presence of silicon rich oxide films very detrimental to corrosion resistance.
In tin-containing prealloyed stainless steel powders, they observed the surfaces of both powders and sintered parts to be highly enriched with tin which lead to improved corrosion resistance in highly compacted bodies. (Tin modified 304L parts showed less susceptibility to chrome nitride formation than regular 304L.).
The corrosion resistance of stainless steel powder metal parts, particularly low density porous metal parts, e.g., filters, is poor as compared to their wrought counterparts. One explanation for this behavior has been the inherent porosity of parts, which provides sites for crevice corrosion to occur. The presence of pores alone in powder metallurgy stainless steel parts cannot completely explain the inferior corrosion resistance, however, as evidenced by the fact that stainless parts processed and/or sintered under different conditions but having the same sintered density may have corrosion resistances differing by one to two orders of magnitude.
Another explanation is the metallurgical impact of the high-temperature sintering cycle which the part undergoes. There are several parts to this impact. The first concerns the `sensitization` of the part by the precipitation of chromium carbides at the grain bondaries during cooling through the sensitization range of 1400.degree. to 950.degree. F. The region adjacent to the grain boundary is denuded by chromium by this carbide precipitation, and therefore, is susceptible to corrosion before the remainder of the part. This is the reason that powder metallurgy stainless powders are of the `L` or low carbon grades, and are not sintered in carbon-containing atmospheres.
Sintering also reduces oxides from the original powder surfaces. Silicon oxides, which normally exist on water atomized powder surfaces, are broken during compaction of parts. The broken films may provide sites for crevice corrosion to initiate. It is believed that sintering in a reducing atmosphere causes partial removal of the fragmented films. The films may or may not be replaced by whole protective films. The degree to which oxide reduction occurs depends on the dewpoint, temperature, and reducing power (hydrogen partial pressure) of the sintering gas, as in a dissociated ammonia gas atmosphere.
It has been shown that the presence of tin in a stainless steel powder, either blended or prealloyed, leads to superior corrosion resistance of vacuum sintered parts (see Japanese Pat. No. 35708/1977 by Tatsua Hisada) and of parts sintered in hydrogen or dissociated ammonia (see `Corrosion Behavior of P/M Austenitic Stainless Steels` by D. H. Ro and E. Klar (supra). Hisada explained the beneficial effect of tin as (1) heightening the corrosion resistance of the matrix alloy and (2) improving the compressibility of the powder. Ro and Klar, on the other hand, attributed the benefits of tin as the formation of chemically more stable passive films in a crevice.
It has now been shown that the tin effect is much more far reaching than previously believed. Parts of low carbon content are still susceptible to a sensitization phenomenon. This phenomenon is caused by the precipitation of chromium nitrides in the grain boundaries. The denuding of chromium adjacent to grain boundaries occurs just as in the carbide sensitization case.
The problems discussed above are particularly pronounced in connection with low density porous sintered stainless steel parts, e.g., filters, and as will be shown below the improvement in corrosion resistance obtained with tin and tin-copper alloys is unexpected under the circumstances. Although the invention is useful in all low density powdered stainless steel parts, it is of primary value in filter media of any size. These media have a density less than 80% of theoretical, and preferably less than 70% of theoretical where the sharply increased resistance to corrosion is clearly observed.